1. Field of the Invention
The present invention relates to a semiconductor device as typified by a thin-film transistor formed on a substrate, as well as to a manufacturing method thereof.
2. Description of Related Art
The thin-film transistor using a silicon film is known. In this technique, a thin-film transistor is formed by using a silicon film formed on a glass or quartz substrate.
The reason for using a glass or quartz substrate is that thin-film transistors are used for an active matrix liquid crystal display device. Conventionally, thin-film transistors are formed by using an amorphous silicon film. However, to provide higher performance, it is now attempted to form thin-film transistors by using a silicon film having crystallinity (called a crystalline silicon film).
A thin-film transistor using a crystalline silicon film can operate faster than that using an amorphous silicon film by two or more orders. Therefore, by employing thin-film transistors using a crystalline silicon film, peripheral drivers circuits of an active matrix liquid crystal display device, which are conventionally external IC circuits, can be formed on a glass or quartz substrate with an active matrix circuit.
The above configuration is very advantageous in terms of miniaturization of the entire device and simplification of a manufacturing process as well as reduction in manufacturing cost.
A crystalline silicon film is obtained by forming an amorphous silicon film through plasma CVD or low-pressure thermal CVD, and then crystallizing it through a heat treatment or irradiation with laser light.
However, with the heat treatment, it is currently difficult to obtain a desired level of crystallinity over a wide area; for example, crystallization is uneven.
On the other hand, although the method using the laser light irradiation can provide superior crystallinity partially, it is difficult for even such a technique to provide good annealing effects over a wide area. In particular, irradiation with laser light under such conditions as to provide a high degree of crystallinity tends to be unstable.
A technique described in Japanese Unexamined Patent Publication No. Hei. 6-232059 is known as a method for solving the above problems. In this technique, a metal element (for instance, nickel) for accelerating crystallization of silicon is introducing an amorphous silicon film, thereby providing a crystalline silicon film by a heat treatment of a lower temperature than in conventional techniques.
Studies of the present inventors have proved that a crystalline silicon film produced by this technique has, over a wide area, sufficiently high crystallinity to be suitable for practical use.
However, this technique has a problem that delicate control is needed for the introduction amount of a metal element because it remains in a resulting film. Accordingly, it has been found that this technique is problematic in reproducibility and electrical stability of a device produced.
In addition, there is a problem that the characteristics of a semiconductor device produced considerably varies with time, which is an influence of a residual metal element. The residual metal element also causes a problem that the off-current of a thin-film transistor fabricated by using the above-mentioned film is large.
That is, although the metal element for accelerating crystallization of silicon is very useful to form a crystalline silicon film, it is associated with negative factors that cause various problems after formation of a crystalline silicon film.